1. Field of the Invention
The present invention relates to a method of cast-welding between finely pearlitized rails by pouring aluminothermically produced steel into a casting mold which surrounds the ends of the two rails.
The invention particularly relates to a method of cast-welding between finely pearlitized rails in which the rail head parts adjacent both sides of the welding seams have an increased strength which is approximately the same as that of the finely pearlitized rails.
2. Description of the Related Art
Under a given load during railroad operations, the wear of a rail is predominantly determined by the rail strength or hardness. At the present time, for continuously welded rails, the railroads utilize mostly naturally hard rails having a minimum tensile strength of 900 N/mm.sup.2. For obtaining the rail strength, the alloy elements carbon and manganese are available to the rail manufacturers. In the case of substantially increased loads, as they occur, for example, in outer rails in curves, the naturally hard special grade steel which is additionally alloyed with chromium and/or vanadium and has a minimum tensile strength of 1100 N/mm.sup.2 is also used.
As an alternative to the naturally hard alloyed speoial grade steel, it is also possible to obtain high strengths by means of a heat treatment of the rails after the rolling procedure. The heat treatment is usually limited to the part of the rail near the tread. The chemical composition of such rails corresponds approximately to the rails having a minimum tensile strength of 900 N/mm.sup.2, wherein the pearlitic structure is extremely finely lamellar due to the heat treatment, resulting in an appropriately high hardness or strength. The present invention is directed to improving the hardness patterns of aluminothermically produced welds in the head parts of this type of rail.
The aluminothermic welding process is a fusion welding process and leads to the characteristic formation of the welding area in the form of an intermediate cast portion which is composed of aluminothermically produced steel and dissolved rail steel and which is located centrally in the originally present gap between the two ends of the rails, and a heat-influenced zone to the right and left of the welding seams.
FIG. 1a of the drawing shows the above-described portions in a schematic longitudinal sectional view through the plane of symmetry of a finely pearlitized (head-hardened) rail. Reference numeral 1 denotes the intermediate cast portion composed of aluminothermically produced steel and melted rail steel. The two heat-influenced zones 3 border the intermediate cast portion 1 at weld lines 2. Rail steel 4 which has not been influenced by heat is located adjacent heat-influenced zones 3. Reference numeral 5 denotes the finely pearlitized rail head parts.
The above-described configuration is extremely important for the wear behavior during railroad operations. The hardness and, thus, the strength of the welding seam and, consequently, the wear resistance can be influenced rather precisely by the freely selectable chemical composition of the aluminothermic steel, when the cooling speed of the weld is predetermined. Accordingly, the hardness of the intermediate cast portions represents no problems for rail welding with respect to wear technology. However, the situation is different within the two heat-influenced zones. In the heat-influenced zones, the given chemical composition of the rail and the cooling rate of the weld together determine the hardness distribution. Within the heat-influenced zones, the hardness increases seen in longitudinal direction of the rails with increasing distance from the center of the welding seam, until a minimum value is passed at the border between the heat-influenced zone and the rail steel which is not influenced by heat. This is where the so-called soft-annealed zone is located. Since the resistance to wear also decreases with decreasing hardness, an increased wear, particularly in the heat-influenced zones, must be expected during railroad operations.
The increased hardness within the heat-influenced zones is due to metal/physical reasons. The rail steel is austenitized in the greatest portion of the heat-influenced zones adjacent the intermediate cast portions. On the other hand, in the portion of the heat-influenced zones remote from the welding seam, a maximum temperature of 600.degree.-700.degree. C. is reached in the rail. During cooling of the weld, different structures are formed in the heat-influenced zones:
coarse-grained, hard pearlite at the weld line, i.e., at the transition between intermediate cast portion and heat-influenced zones; PA1 spheroidite soft globular pearlite at the transition between the heat-influenced zones and the rail steel which is not influenced by heat, i.e., at the end of the heat-influenced zone. PA1 a) The width of the gap between two rail ends to be welded together is set at 1 to 2 times the thickness of the web of the rails; PA1 b) the shape of the casting mold is selected in such a way that the welding bead has a width of 2 to 4 times the thickness of the web of the rail and a depth of 0.15 to 0.6 times the thickness of the web; PA1 c) the width of the ribbon-shaped flame of the burner for preheating is set at 1.5 to 2.5 times the thickness of the web of the rails.
Since the cooling rate of the aluminothermic weld and the chemical composition of the rails is predetermined, a certain structure is obtained depending on the quality of the rail steel and, thus, a certain hardness distribution in the heat-influenced zones seen in longitudinal direction of the rail is obtained.
When welding naturally hard rails, i.e., rails which were not subjected to heat treatment by the manufacturer after rolling, the hardness distribution within the heat-influenced zones depends on the chemical composition of the rail steel. The alloy elements C, Mn, V, Cr and others influence the hardness level through the transformation behavior of the rail steel and/or through the formation of carbide. However, the hardness of rails which have not been influenced by heat are also only controlled through these two mechanisms during cooling after the rolling procedure. Thus, when such naturally hard rails are welded, always approximately the same difference results between the hardness of the non-uniform rail and the hardness pattern in the heat-influenced zones, independently of the rail steel analysis. From the viewpoint of wear technology, this difference can be tolerated in such aluminothermical welds.
In the case of finely pearlitized rails which are heat-aftertreated, in which the hardness level is "artificially" raised after rolling, a greater difference occurs in the hardness because the original finely lamellar pearlite structure is destroyed and the hardness distribution in the heat-influenced zones after welding automatically is that of a welded naturally hard rail having minimum tensile strength of 900N/mm.sup.2. This is due to the same chemical composition of the two rail steel grades.
FIG. 1b shows the respective hardness patterns at the tread seen in longitudinal direction of the rail. Curve I corresponds to the hardness pattern of a weld of finely pearlitized (head-hardened) rails and curve II corresponds to the hardness pattern of a weld of naturally hard rails having a minimum tensile strength of 900N/mm.sup.2. Compared to the welding material and the non-influenced rails, the tread of the finely pearlitized rails is subjected in the regions of the heat-influenced zones to a relatively stronger wear than is the case in a weld of naturally hard rails. For this reason, welding technology for finely pearlitized rails must meet higher requirements.
In practice, two methods of aluminothermically cast-welding between rails have been found particularly useful and are used predominantly. The methods are those disclosed in British patent 1,402,964 and U.S. Pat. No. 3,942,579.
The method according to the British patent 1,402,964 includes the combination of the following steps:
When carrying out rail welding by means of the above-described method, a preheating period for the rail ends surrounded by the mold halves of approximately 6 to 10 minutes is required.
The method according to U.S. Pat. No. 3,942,579 includes preheating the rail ends to be welded together within a period of time of at most 2 minutes to a temperature of between a minimum of 300.degree. C. and a maximum of 700.degree. C., wherein the weight of the aluminothermic mixture is 0.15 to 0.25 parts by weight of the weight per meter of the rails to be welded together. This method is considered a welding process with short preheating.
Compared to methods using normal preheating, the steep temperature gradient in longitudinal direction of the rails resulting from short preheating leads to a smaller expansion of the heat-influenced zones. In this method, the drop of hardness in the heat-influenced zones is limited to a smaller area seen in longitudinal direction of the rails. This has a positive effect on the wear behavior during railroad operations. For this reason, this method is particularly preferred for the process of the present invention.
The configuration of the intermediate cast portion and of the heat-influenced zones of such a weld can be influenced by modifying the design of the casting mold. For example, it is possible to obtain a greater dissolution of the rail ends and, thus, a greater intermediate cast portion seen in longitudinal direction of the rail in a layer underneath the tread which is not too thick. This results in an approach of the weld line to the rail in the area of the tread which has not been influenced by heat and which solely determines the wear of the rail and of the weld. However, by utilizing a suitable casting technology, it is possible to maintain unchanged the configuration of the intermediate cast portion and of the heat-influenced portions and of the heat-influenced zones in the remaining cross-section of the rail, so that it is possible to maintain unchanged the configuration of the intermediate cast portions and of the heat-influenced zones in the remaining cross-section of the rail, so that the outer limits of the heat-influenced zones are "held" at the running surface. In this case, the heat-influenced zones are advantageously made smaller at the running surface.
An improvement of the hardness pattern in the heat-influenced zones can be obtained in a known manner by an additional heat treatment of the rails in the welding area. Independent of whether an aluminothermical welding procedure or a flash butt-welding procedure is carried out, such a heat treatment is conventionally carried out when the weld has cooled to ambient temperature or at least to a temperature below 700.degree. C., i.e., to a temperature at which the austenite/pearlite transformation has concluded. The weld is again austenitized by means of a suitable gas burner device and is subsequently cooled in an accelerated manner by applying compressed air or a mixture of compressed air/water or a cooling medium having the same effect. However, this method has the disadvantage that it is difficult to carry out on rails which have already been laid and that additionally a complicated apparatus is required which must be transported to the construction site.